dead cylinder

sold my xj 700 we became friends, dude was in behind him said big puff black smoke from right side.. i was there tonight, all 3 pipes are hot,, 4th pipe i wrapped hand around,, its cold.. dead cylinder, she was runnin fine before, dropped a cylinder,, i checked spark,,its getting sprk from plug wire to spark plug when turned over any clue guys? i was thinkin coil,,but theres spark, #4 cylinders dead,. wet spark,, cold,,,

 

i should add theres no white or blue smoke.

 

Compression?

 

Yeah, start there. Do a compression test; un-compressed mixture won't light.

 

And while the plugs are out for the test, throw them away & fit new ones.
Modern fuel isn't kind to plugs and they can appear to spark fine when "tested" against the side of the head at atmospheric pressure and with no mixture to ignite, but can break down and stop sparking once they are under compression & surrounded by fuel/air mix.

Added to the fact they're dirt cheap, they may as well be ruled out straight away. (also make sure they're the correct kind, non-resistor type with resistor plug caps and vice-versa)

 

he doesnt have a comp tester, but its cold pipe,,all other 3 i couldnt even touch,

 

i told him dont even start it anymore and to drain oil and check for metal filings,

 

i feel bad for the guy,,

 

you guys know this bike,,i complained enuff all season lol..i got it runnin good,,then i sell it and it blows up,,

 

sounds like you got rid of it just in time

 

lookin for a cause here always a reason,

 

I'm sure there are auto parts stores in Canada that "loan" or rent tools just like here in the States. Or buy one, it's a whole $25 investment.

If it dropped a valve or holed a piston it won't produce a combustible "charge." No fire = no heat. Many possibilities for no fire.

If the compression is good then you start looking for other causes.

DON'T ASSUME. DIAGNOSE.

Obviously that cylinder isn't firing. There could be any number of reasons for that, but to diagnose it you have to go in order. And that begins with making sure it can actually produce compression in that cylinder. If it CAN, then you move to the next step. If not, then cry the blues.

 

Where are you in Ontario? I could help you if your close.

 

Just take the plug out and hold your thumb on the hole.
Crank it over a few times and see if you can hold back the compression.

 

That "kinda" works; except 45psi will quickly pop your thumb out of the hole but anything less than about 60psi ain't gonna fire.

 

How do you make 10 psi with a valve that don't work or a hole in in the piston

 

ok talked to buddy, hes gunna rent a comp tester,,but thumb over hole could sorta be a good easy step for now,, but i know earlier this season the comp was 4 psi off stock,

 

im in kitchener btw

 

ok he just did that thumb test,, he said his thumb got blown off the hole np,

 

As Polock pointed out, a cylinder with a dumped valve or holed piston won't; however a BURNT valve (or other compression-reducing situation) may still cause low enough compression for no combustion, even if it does pop your thumb off the hole.

My point being, the "thumb on the hole" test is NOT necessarily proof of enough compression for combustion to occur.